Double diffused MOS transistor and method for manufacturing same

ABSTRACT

A method of manufacturing a semiconductor device, such as a double-diffused metal oxide semiconductor (DMOS) transistor, where a first layer may be formed on a semiconductor substrate, with isolation trenches formed in the first layer and semiconductor substrate, and with the trenches being filled with an isolation layer. A second layer may be formed on the first layer and semiconductor substrate, and a plurality of drain trenches may be formed therein. A pair of plug-type drains may be formed in the trenches, to be separated from the isolation layer by a dielectric spacer. Gates and source areas may be formed on a resultant structure containing the plug-type drains. Accordingly, current may be increased with a reduction in drain-source on resistance, and an area of the isolation layer can be reduced, as compared to an existing isolation layer, potentially resulting in a reduction in chip area.

CROSS-REFERENCE TO RELATED CASES

This application claims the priority of Korean Patent Application No.2002-0043697, filed Jul. 24, 2002, in the Korean Intellectual PropertyOffice, the disclosure of which is incorporated herein in its entiretyby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a power device and a method formanufacturing the same.

2. Description of the Related Art

Double-diffused metal oxide semiconductor (DMOS) transistors, which areMOS-type transistors driven by voltage and which may bear high current,have begun attracting greater attention. DMOS transistors integratedwith bipolar-type integrated circuits may be classified as lateral DMOS(LDMOS) transistors, or vertical DMOS (VDMOS) transistors, depending onthe direction of current flowing therein. In particular, for DMOStransistors operating at a high voltage, it is advantageous that theDMOS transistors be VDMOS transistors because VDMOS transistors occupysmaller area on a substrate than LDMOS transistors. Among VDMOStransistors, N-channel VDMOS transistors, which have sound electricalcharacteristics, are typically used in bipolar-type integrated circuits.

N-channel VDMOS transistors are connected to N+ buried layers, using theN+ buried layers as drains. The drains may be heavily doped withimpurity ions, thus isolation layers with considerable surface areas arerequired in order to isolate VDMOS transistors from each other. In acase where doping concentration is increased and drive-in time (e.g.,time necessary to drive dopant atoms deeper into a semiconductor waferof the device) long, a large-sized semiconductor device with a widerisolation layer may be required in order to reduce a drain-sourceon-resistance (Rdson), so as to maintain breakdown voltage of thesemiconductor device. Where high-dose ion implantation is used forreducing Rdson, a thermal process requiring a sufficient amount of heatmay be needed to diffuse impurity ions into a lower portion of thesemiconductor device.

Due to the increased heat required, the area of the semiconductor devicemay increase in a horizontal direction during the diffusion.Additionally where high-energy ion implantation is used to reduce Rdson,since high-dose doping may be difficult, high-energy ion implantationtechniques may be limited when reducing Rdson, since these techniquescannot avoid the diffusion of impurity ions in a vertical direction.Thus, high-energy ion implantation techniques may be disadvantageous tothe scaling of the semiconductor device. As a result, a trade-off mayexist between a desire to reduce Rdson, and a desire to reduce an areaof an isolation layer.

However, the need for mounting small-sized, highly-integratedsemiconductor devices that consume a small amount of power on asemiconductor chip continues to increase. Thus, a technique forachieving appropriate performance from semiconductor devices by reducingRdson and scaling down the semiconductor devices along horizontal andvertical dimensions is desired.

SUMMARY OF THE INVENTION

Exemplary embodiments of the present invention provide a method formanufacturing a semiconductor device, such as a DMOS transistor, forexample, by which Rdson and the size of a semiconductor chip containingthe device may be reduced. Exemplary embodiments of the presentinvention also may provide a DMOS transistor which has a small Rdson,which may be advantageous in the integration of a semiconductor device.

In an exemplary embodiment, the present invention is directed to amethod of forming a semiconductor device, where a first layer may beformed on a semiconductor substrate, and one or more isolation trenchesmay be formed and filled with an isolation layer. A second layer may beformed on the first layer and semiconductor substrate, and a pluralityof drain trenches may be formed therein. A pair of plug-type drains maybe formed in the trenches, to be separated from the isolation layer by adielectric spacer. Gates and source areas may be formed on a resultantstructure containing the plug-type drains. Accordingly, current may beincreased with a reduction in drain-source on resistance, and an area ofthe isolation layer can be efficiently reduced, as compared to anexisting isolation layer, potentially resulting in a reduction in chiparea.

According to a further exemplary embodiment, the present invention isdirected to a method of manufacturing a double-diffused metal oxidesemiconductor (DMOS) transistor. A buried layer containing impurity ionsof a second conductivity type may be formed in a semiconductor substratecontaining impurity ions of a first conductivity type. Isolationtrenches may be formed in the buried layer and semiconductor substrate,and an isolation layer may be formed in the isolation trenches. Anepitaxial layer containing impurity ions of the second conductivity typemay be grown on the buried layer and isolation layer, and drain trenchesmay be formed in isolation layer. A dielectric spacer may be formed onsidewalls of the drain trenches, and plug-type drains containingimpurity ions of the second conductivity type may be formed on thedielectric spacer within the drain trenches. Gates and source areas maybe formed on a resultant structure containing the plug-type drains.

According to another exemplary embodiment, the present invention isdirected to a double-diffused metal oxide semiconductor transistor thatmay include a semiconductor substrate, a buried layer, an isolationlayer, an epitaxial layer, plug-type drains, a dielectric spacer, bodyareas, source areas, and gate oxide layers and gates. The semiconductorsubstrate may include impurity ions of a first conductivity type. Theburied layer may be formed on the semiconductor substrate and includeimpurity ions of a second conductivity type, different from the firstconductivity type. The isolation layer may be formed in the buried layerand semiconductor substrate. The epitaxial layer may be formed on theisolation layer and buried layer and may include impurity ions of thesecond conductivity type. The plug-type drains may be formed in theepitaxial layer and may include impurity ions of the second conductivitytype. The dielectric spacer may be formed on outer sidewalls of theplug-type drains adjacent to the isolation layer. The body areas may beformed in the epitaxial layer and include impurity ions of the firstconductivity type. The source areas may be formed in the body areas andinclude impurity ions of the second conductivity type, and the gateoxide layers and gates may be formed on the source areas.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of thehe present invention will become more fullyunderstood from the detailed description given hereinbelow and theaccompanying drawings, wherein like elements are represented by likereference numerals, which are given by way of illustration only and thusare not limitative of the present invention and wherein:

FIGS. 1 through 6 are cross-sectional views describing a method ofmanufacturing a VDMOS transistor according to an exemplary embodiment ofthe present invention; and

FIGS. 7 and 8 are cross-sectional views describing a method ofmanufacturing a VDMOS transistor according to another exemplaryembodiment of the present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, exemplary embodiments of the present invention will bedescribed in detail with reference to the attached drawings. However,the exemplary embodiments of the present invention can be modified intovarious other forms, and the scope of the present invention must not beinterpreted as being restricted to the exemplary embodiments. Theexemplary embodiments are provided to more completely explain thepresent invention to those skilled in the art. In drawings, thethicknesses of layers or regions are exaggerated for clarity. Likereference numerals in the drawings denote the same members. Also, whenit is written that a layer is formed “on” another layer or a substrate,the layer can be formed directly on the other layer or the substrate, orother layers can intervene therebetween.

FIGS. 1 through 6 are cross-sectional views describing a method ofmanufacturing a VDMOS transistor according to an exemplary embodiment ofthe present invention. First, referring to FIG. 1, a semiconductorsubstrate (P-sub) 10, such as a P-type single crystal silicon substrate,may be prepared. For example, a two-step diffusion sequence may be usedto form active p-n junctions in single crystal silicon wafers bydistributing controlled amounts of electronically active diffusants, or“dopants.” The starting wafer, such as semiconductor substrate 10, mayinitially contain a substantially uniform concentration of dopant. Inthe first step, called “pre-deposition diffusion,” a controlled amountof the dopant may be introduced into the semiconductor substrate 10 neara free surface. In the second step, called “drive-in diffusion,” theinitially pre-deposited source of dopant may be diffused much moredeeply into the semiconductor.

Thus, to prepare semiconductor substrate 10, N-type impurity ions may beimplanted at a concentration of about 10¹⁹ ions/cm³ into semiconductorsubstrate 10. The drive-in diffusion step may then be performed todiffuse the implanted impurity ions. A N+ buried layer (N+ BL) 15 may beformed on the semiconductor substrate 10 once the drive-in diffusionstep is completed.

The N+ buried layer 15 and the semiconductor substrate 10 may then beetched to form an isolation trench 20. The isolation trench 20 may befilled with an insulating layer to form an isolation layer 25. Theinsulating layer may be comprised of an oxide or undoped silicon, forexample.

The isolation layer 25 may be formed as follows. For example, a padoxide layer (not shown) and a pad nitride layer (not shown) may beformed on the buried layer 15 and patterned to make an opening forforming the isolation trench 20. The buried layer 15 and semiconductorsubstrate 10 may be etched using the patterned pad oxide layer andpatterned pad nitride layer as masks, forming the isolation trench 20 toa depth of several thousands Å, e.g., several micrometers (μm), forexample.

Thereafter, the isolation trench 20 may be filled with oxide or undopedpolysilicon. The oxide may be an oxide (e.g., middle temperature oxide(MTO)) formed by using a low pressure chemical vapor deposition (LPCVD)process such as a middle temperature oxide process, or an oxide formedusing a high-density plasma chemical vapor deposition (HDP-CVD) process,for example, or other known deposition processes. The undopedpolysilicon may be deposited at a temperature of 500° C.-700° C. usingLPCVD or other known deposition processes, for example. Also, before theoxide or the undoped polysilicon is deposited, a thermal oxide may beformed on an inner wall of the isolation trench 20, so as to stabilizean etch interface between the isolation trench 20 and the buried layer15, and/or between the isolation trench 20 and the semiconductorsubstrate 10. An upper surface of the resultant structure may beplanarized using chemical mechanical polishing (CMP) or etch back.During CMP or etch back, the patterned pad nitride layer may operate asa stopper. The pad nitride layer and the pad oxide layer remaining onthe buried layer 15 may then be removed to form an isolation layer 25that may be slightly higher than or substantially parallel to thesurface of the buried layer 15, for example.

Referring to FIG. 2, an N-epitaxial layer (N-epi) 30 may be grown on theburied layer 15 and the isolation layer 25, and etched to a depth thatreaches the buried layer 15, thereby forming drain trenches 41 and 42over both sidewalls of the isolation layer 25. The epitaxial layer 30may have an impurity concentration of about 10¹⁵ ions/cm³, for example.

As shown in FIG. 3, a dielectric layer 45 may be deposited on theresultant structure shown in FIG. 2 in order to form dielectric spacersin the drain trenches 41 and 42, on sidewalls 41 a and 42 a above theisolation layer 25. The dielectric layer 45 may be deposited tothickness that does not completely fill the drain trenches 41 and 42,and may be embodied as a silicon nitride layer or a silicon oxide layer,for example. A dielectric layer 45 of silicon nitride may be depositedusing plasma-enhanced CVD (PECVD) in which SiH₄ and NH₃ may be used assource gases, and in which Ar and/or He may be used as a carrier gas,for example. Exemplary embodiments of the present invention are notlimited to the above deposition process, however, as other knowndeposition processes may be substituted for PECVD.

A mask 50 may be formed to protect portions of the dielectric layer 45that are formed on the sidewalls 41 a and 42 a. For example, aphotoresist may coat a resultant structure on which the dielectric layer45 is formed, and then may be exposed and developed to leave a patternedphotoresist on only the portion of the dielectric layer 45 that isformed on sidewalls 41 a and 42 a.

As shown in FIG. 4, a portion of the dielectric layer 45 not protectedby the mask 50 may be etched back using CFx, CHxFy, or Hx as an etchgas, for example. Thus, the portion of the dielectric layer 45 protectedby the mask 50 may remain to form a spacer 45 a on the sidewalls 41 aand 42 a of drain trenches 41 and 42. The spacer 45 a may cover aportion of a surface of the epitaxial layer 30 between drain trenches 41and 42, as shown in FIG. 4.

Referring to FIG. 5, the mask 50 may be removed to expose the spacer 45a, and then doped polysilicon may be deposited so as to cover theepitaxial layer 30, and so as to fill drain trenches 41 and 42. Thedoped polysilicon may be deposited at a temperature of 500° C.-700° C.using LPCVD, for example. The undoped polysilicon may be deposited anddoped with arsenic (As) or phosphorus (P) using an ion implantationmethod, for example. Alternatively, undoped polysilicon may be dopedwith impurities in-situ and deposited using LPCVD, for example. In anexemplary embodiment, the doped polysilicon may contain impurity ions of10¹⁴ ions/cm³ or more. The impurity ions used in the doping may be oneor more of As, P, B, In, Sb, etc. The impurity ions selected may dependon the conductivity type of a VDMOS transistor that is to bemanufactured. In an exemplary embodiment, if an N-channel VDMOStransistor is to be manufactured, N-type impurity ions such as As, P,Sb, etc., may be used. Portions of the doped polysilicon covering theepitaxial layer 30 may be etched back so that the doped polysiliconremains only in the drain trenches 41 and 42. As a result, plug-typedrains 60, which fill the drain trenches 41 and 42, may be formed in theepitaxial layer 30, as shown in FIG. 5.

FIG. 6 illustrates gates and sources formed on the structure of FIG. 5.Local oxidation of silicon (LOCOS) layers 100 may be formed at a surfaceof the epitaxial layer 30, and then P-type body areas 110, having animpurity concentration of about 10¹⁸ ions/cm³, for example, may beformed in specified portions of the epitaxial layer 30, such as thelocations shown in FIG. 6, for example. The LOCOS layers 100, andthermal oxide layers (not shown) formed on and/or around the LOCOSlayers 100 are used as gate oxide layers. Gates 130 may be formed on thethermal oxide layers. N+ source areas 115 having an impurityconcentration of about 10¹⁹ ions/cm³, and P+ channel areas 120 having animpurity concentration of about 10¹⁹ ions/cm³ may be formed in theP-type body areas 110, by known processes. The gates 130 may beconnected to gate electrodes (gate), the N+ source areas 115 and the P+channel areas 120 may be connected to source electrodes (source), andthe plug-type drains 60 may be connected to drain electrodes (drain),for example.

As described above, the drains 60 may be formed by burying a conductivematerial, instead of by implanting ions. Since the diffusion of impurityions at junctions due to a subsequent thermal process can be reducedand/or eliminated, and desired heavy doping is possible, drainresistance may be reduced. If the drains 60 are formed of a conductivematerial having a low resistance, Rdson may be reduced to a desiredlevel. Also, since the spacer 45 a is formed near the isolation layer25, isolation may be improved as compared to a transistor having onlythe isolation layer 25. Thus, even though the area of the isolationlayer 25 is small, sound isolation effects may be possible. Therefore,area reduction of the isolation layer 25 may be advantageous in theproduction of high density semiconductor devices.

Referring to FIG. 6, a VDMOS transistor according to exemplaryembodiments of the present invention may include a semiconductorsubstrate 10 containing impurity ions of a first conductivity type,i.e., P-type ions, and the buried layer 15 containing impurity ions of asecond conductivity type, i.e., N-type ions. The buried layer 15contains impurity ions of the second conductivity type at a specifiedfirst concentration (e.g., N-type impurity ions implanted at aconcentration of about 10¹⁹ ions/cm³).

The isolation layer 25 may be formed in the buried layer 15 and thesemiconductor substrate 10. The epitaxial layer 30, containing impurityions of the second conductivity type, but at a second concentration thatis lower than the first concentration (e.g., N-type impurity ionsimplanted at a concentration of about 10¹⁵ ions/cm³), may be formed onthe isolation layer 25 and the buried layer 15.

The plug-type drains 60 may be formed of a conductive material (in thisembodiment, doped polysilicon) to a depth reaching the buried layer 15,through epitaxial layer 30 and over both sidewalls of the isolationlayer 25. A spacer 45 a may be formed on outer walls of the drains 60adjacent to the isolation layer 25, so as to help isolation amongdevices.

The body areas 110 containing impurity ions of the first conductivitytype may be formed in the epitaxial layer 30. The source areas 115containing impurity ions of the second conductivity type, and at aconcentration substantially equal to the first concentration, may beformed in body areas 110. Channel areas 120 containing impurity ions ofthe first conductivity type (e.g., P-type impurity ions), and alsosubstantially equal to the first concentration, may be included betweenthe body areas 110 and the source areas 115. The LOCOS oxide layers 100used as gate oxide layers may be formed beside the source areas 115 andhigher in relation to the source areas 115, and the gates 130 may beformed over the source areas 115.

The gates 130 may be connected to gate electrodes (gate), the sourceareas 115 and the channel areas 120 may be connected to the sourceelectrodes (source), and the drains 60 may be connected to the drainelectrodes (drain). Channels may be formed on a surface of the channelareas 120, due to a voltage applied to the gates 130. Thus, a currentflows from the drains 60 through buried layer 15, through the epitaxiallayer 30 under the gates 130, and through the channels into the sourceareas 115.

As described above, the area of an isolation layer and Rdson can bereduced, and thus a packing density of a semiconductor device may beincreased, providing a potential for improved semiconductor device(e.g., transistor) performance with reduced maintenance. Also, unitdevices on the semiconductor device maybe more efficiently isolated.

FIGS. 7 and 8 are cross-sectional views describing a method ofmanufacturing a VDMOS transistor according to another exemplaryembodiment of the present invention.

In FIGS. 7 and 8, the same elements as those described in the previousexemplary embodiment denote the same reference numerals as those shownin FIGS. 1 through 6. Hereinafter, only steps that are different fromsteps described in the previous exemplary embodiments will be described.

Steps described with reference to FIGS. 1 through 5 in the previousexemplary embodiments are similar to those of this exemplary embodiment.In other words, doped polysilicon is deposited so as to cover theepitaxial layer 30 and completely fill the drain trenches 41 and 42shown in FIG. 5.

As shown in FIG. 7, upper portions of the doped polysilicon may beadditionally doped with POCl₃. Thus, the upper portions of the dopedpolysilicon in the drain trenches 41 and 42 become low resistive layers70 in which sheet resistance may be reduced to a resistance valuebetween about 5 Ω/sq to 50 Ω/sq. In the previous exemplary embodiments,the doped polysilicon may be a low resistive material which is dopedwith impurity ions to a concentration of 10¹⁴ ions/cm³ or more. However,where the resistance is desired to be further reduced, e.g., based oncertain device requirements, the process of doping phosphorusoxychloride (POCl₃) may be additionally performed. Thereafter, portionsof the doped polysilicon remaining on the epitaxial layer 30 may beetched back. As a result, plug-type drains 60 having low resistivelayers 70 doped with POCl₃ may be formed in drain trenches 41 and 42.

Referring to FIG. 8, gates and source areas may be formed on theresultant structure shown in FIG. 7. Detailed descriptions are the sameas those explained with reference to FIG. 6, and thus will not berepeated here.

As described above, in this exemplary embodiment, doped polysilicon maybe deposited and additionally doped with POCl₃ so as to form the drains60. This may result in a marked reduction in Rdson.

In the exemplary embodiments of the present invention, a conductivematerial is buried in an epitaxial layer to form plug-type drains. Theconductive material may be heavily-doped polysilicon or heavily-dopedpolysilicon additionally doped with POCl₃. Unlike the prior art, ionimplantation and ion diffusion are not used or performed in order toform drains, and thus the diffusion of ions into junctions, due to asubsequent thermal process, can be reduced, while desired heavy dopingmay still be possible. Thus, Rdson may be more easily reduced, ascompared to conventional processes, which may result in an increase incurrent through the device.

Also, since a dielectric spacer is additionally formed near or in thevicinity of an isolation layer, and although the formed isolation layeris smaller than an existing isolation layer of a conventionaltransistor, an improved isolation effect may be expected. As a result,reduction in the area of the isolation layer may enable scaled downsemiconductor chips. Further as described in the above exemplaryembodiments, resistance of the plug-type drains can be reduced to adesired level by using a conductive material having a low resistance.This too may be advantageous to the high integration of semiconductordevices.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the present invention as defined by the following claims.

What is claimed is:
 1. A method of manufacturing a double-diffused metaloxide semiconductor (DMOS) transistor, comprising the steps of: (a)forming a buried layer containing impurity ions of a second conductivitytype in a semiconductor substrate containing impurity ions of a firstconductivity type; (b) forming isolation trenches in the buried layerand the semiconductor substrate; (c) forming an isolation layer in theisolation trenches; (d) growing an epitaxial layer containing impurityions of the second conductivity type on the buried layer and theisolation layer; (e) forming drain trenches in the isolation layer; (f)forming a dielectric spacer on sidewalls of the drain trenches; (g)forming plug-type drains containing impurity ions of the secondconductivity type on the dielectric spacer within the drain trenches;and (h) forming gates and source areas on a resultant structurecontaining the plug-type drains.
 2. The method of claim 1, wherein step(b) further comprises etching the buried layer and semiconductorsubstrate to form the isolation trenches.
 3. The method of claim 1,wherein step (c) further comprises filling the isolation trenches withinan insulating layer composed of at least one of oxide and undopedpolysilicon.
 4. The method of claim 1, wherein step (e) furthercomprises etching the epitaxial layer to a depth that reaches the buriedlayer to form the drain trenches.
 5. The method of claim 1, wherein step(f) further comprises: depositing a dielectric layer on a resultantstructure in which the drain trenches are formed; forming a mask onsidewalls of the drain trenches adjacent to the isolation layer, themask covering portions of the dielectric layer; etching back portions ofthe dielectric layer not covered by the mask; and removing the mask. 6.The method of claim 5, wherein the dielectric layer is one of a siliconnitride layer and a silicon oxide layer.
 7. The method of claim 1,wherein step (g) further comprises filling the drain trenches with aconductive material to form the plug-type drains, the conductivematerial being composed of polysilicon.
 8. The method of claim 1, stop(g) further comprises: depositing doped polysilicon on the epitaxiallayer to fill the drain trenches; and etching back portions of the dopedpolysilicon on the epitaxial layer so that the doped polysilicon remainsonly in the drain trenches.
 9. The method of claim 1, wherein step (g)further comprises: depositing doped polysilicon on the epitaxial layerto fill the drain trenches; doping the doped polysilicon with phosphorusoxychloride (POCl₃); and etching back portions of the doped polysiliconon the epitaxial layer so that the doped polysilicon doped with POCl₃remains only in the drain trenches.
 10. The method of claim 7, whereinthe doped polysilicon has an impurity concentration of at least 1×10¹⁴ions/cm³.
 11. The method of claim 8, wherein the doped polysilicon hasan impurity concentration of at least 1×10¹⁴ ions/cm³.
 12. The method ofclaim 9, wherein the doped polysilicon has an impurity concentration ofat least 1×10¹⁴ ions/cm³.
 13. A method of forming a semiconductordevice, comprising the steps of: forming a first layer on asemiconductor substrate; forming an isolation trench in the first layer;forming an isolation layer in the isolation trench; forming a secondlayer on the first layer and isolation layer; forming at least a pair ofdrain trenches in the second layer; and forming plug-type drains withinsaid pair of drain trenches so that said plug-type drains are separatedfrom said isolation layer by a dielectric spacer.
 14. The method ofclaim 13, further comprising forming gates and source areas on aresultant structure containing the plug-type drains.
 15. The method ofclaim 14, wherein the semiconductor device is embodied as adouble-diffused metal oxide semiconductor (DMOS) transistor.
 16. Themethod of claim 13, wherein the first layer and the second layer eachinclude a concentration of N-type impurity ions, the concentration inthe first layer differing from the concentration in the second layer.17. The method of claim 13, wherein the first layer and the second layereach include a concentration of N-type impurity ions, the concentrationin the first layer greater than the concentration in the second layer.18. A method of manufacturing a double-diffused metal oxidesemiconductor (DMOS) transistor, comprising the steps of: forming afirst layer on a semiconductor substrate; forming an isolation trench inthe first layer; forming an isolation layer in the first trench; forminga second layer on the first layer and isolation layer, the first layerand the second layer each including a concentration of N-type impurityions, the concentration in the first layer differing from theconcentration in the second layer; forming a plurality of drain trenchesin the second layer; forming plug-type drains within the pair of draintrenches so that the plug-type drains are separated from said isolationlayer by a dielectric spacer; and forming gates and source areas on aresultant structure containing the plug-type drains.
 19. The method ofclaim 18, wherein the concentration of the N-type impurity ions in thefirst layer is greater than the concentration of N-type impurity ions inthe second layer.